1. Field of the Invention
The present invention relates to a voltage controlled oscillator, more particuarly, to a voltage controlled oscillator for use in a phase locked loop circuit for generating a cock signal of a semiconductor device, and the like.
2. Description of the Related Art
In general, a voltage controlled oscillator (VCO circuit) comprises a plurality of inverters and transistors. In the VCO circuit, the inverters are connected as a loop, or constitute a loop circuit for generating an oscillation frequency, and the transistors are connected between the inverters and used to regulate characteristics of the oscillation frequency. Namely, the oscillation frequency of the VCO circuit is determined by the sizes of the transistors. Therefore, in a case where there are plural transistors for regulating the characteristics of the oscillation frequency or plural loops (loop circuits) of the VCO circuit, a simulation process for regulating the characteristics of the oscillation frequency becomes complex, so that required characteristics of the oscillation frequency cannot be easily attained. Additionally, as the number of logic circuits (inverter logic circuits) cannot be decreased, an occupancy area thereof becomes large.
It is an object of the present invention to provide a voltage controlled oscillator (VCO circuit) for decreasing the number of loop circuits thereof and simplifying a simulation process to obtain required characteristics of an oscillation frequency.
According to the present invention, there is provided a VCO circuit for controlling an oscillation frequency of an output signal in accordance with a voltage level of an input signal, wherein the VCO circuit comprises: a first inverter logic element, a second inverter logic element, and a third inverter logic element, for constituting a loop circuit thereamong; and a first transistor, a second transistor, and a third transistor, connected between an output of the third inverter logic element and an input of the first inverter logic element, the output signal being taken out from an output of the second inverter logic element, and the oscillation frequency being regulated by the first transistor, the second transistor and the third transistor.
The oscillation frequency of the output signal corresponding to a low voltage range of the input signal may be regulated by the first transistor, the oscillation frequency of the output signal corresponding to a high voltage range of the input signal may be regulated by the second transistor, and the oscillation frequency of the output signal corresponding to a middle voltage range of the input signal may be regulated by the third transistor. The first transistor may be constituted by a P-type MIS transistor, and the second transistor and the third transistor may be constituted by N-type MIS transistors.
The VCO circuit may further comprise a fourth transistor, the input signal may be directly supplied to the first transistor and the second transistor, and be supplied to the third transistor through the fourth transistor. The first transistor and the fourth transistor may be constituted by p-type MIS transistors, and the second transistor and the third transistor may be constituted by N-type MIS transistors.
The VCO circuit may further comprise a first capacitor and a second capacitor for determining the overall range of the oscillation frequency, the first capacitor may be connected between the loop circuit and a first power supply line, and the second capacitor may be connected between the loop circuit and a second power supply line. The first capacitor may connected between the output of the first inverter logic element and the first power supply line, and the second capacitor may be connected between the output of the first inverter logic element and the second power supply line.
The VCO circuit may further comprise a fifth transistor connected between the input of the first inverter logic element and the output of the second inverter logic element. The first transistor and the fifth transistor may be constituted by P-type MIS transistors, and the second transistor and the third transistor may be constituted by N-type MIS transistors.
The first inverter logic element, the second logic circuit and the third logic circuit may be determined as inverter circuits.
The first inverter logic element may be a two-input NAND gate circuit, one input of the first two-input NAND gate circuit may be supplied with an output signal of the third inverter logic element through the first, second and third transistors, and another input of the first two-input NAND gate circuit may be supplied with a control signal for stopping an oscillation operation thereof. The VCO circuit may be applied to a clock signal generator having a crystal oscillator unit, the clock signal generator selects between the output signal of the VCO circuit and an output signal of the crystal oscillator unit and output the clock signal based on the output signals therebetween, and when the output signal of the crystal oscillator unit is selected, the oscillation operation of the VCO circuit may be stopped by the control signal supplied to another input of the first two-input NAND gate circuit.
The second inverter logic element may also be a two-input NAND gate circuit, one input of the second two-input NAND gate circuit may be supplied with an output signal of the first inverter logic element, and another input of the second two-input NAND gate circuit may be supplied with a control signal for stopping an oscillation operation thereof. The VCO circuit may be applied to a clock signal generator having a crystal oscillator unit, the clock signal generator may select between the output signal of the VCO circuit and an output signal of the crystal oscillator unit and output a clock signal based on the output signals therebetween, and when the output signal of the crystal oscillator unit is selected, the oscillation operation of the VCO circuit may be stopped by the control signal supplied to another input of the second two-input NAND gate circuit
According to the present invention, there is also provided a VCO circuit for controlling an oscillation frequency of an output signal by a voltage of an input signal, wherein the VCO circuit comprises: a first inverter logic element; a second inverter logic element connected to an output of the first logic circuit, and the output signal being taken out from an output of the second inverter logic element; a third inverter logic element, connected to an output of the second logic circuit, for constituting a loop circuit together with the first inverter logic element and the second inverter logic element; a first transistor having a control electrode and two other electrodes, for regulating the oscillation frequency of the output signal corresponding to a low voltage range of the input signal, the control electrode of the first transistor being supplied with the input voltage, and the two other electrodes of the first transistor being connected to an output of the third inverter logic element and an input of the first inverter logic element; a second transistor having a control electrode and two other electrodes, for regulating the oscillation frequency of the output signal corresponding to a high voltage range of the input signal, the control electrode of the second transistor being supplied with the input voltage, and the two other electrodes of the second transistor being connected to the output of the third inverter logic element and the input of the first inverter logic element; and a third transistor having a control electrode and two other electrodes, for regulating the oscillation frequency of the output signal corresponding to a middle voltage range of the input signal, the control electrode of the third transistor being operatively supplied with the input voltage, and the two other electrodes of the third transistor being connected to the output of the third inverter logic element and the input of the first inverter logic element.